With the increasing use of radar systems in the civil sector, e.g. in industrial measurement technology (e.g. level measurement), in traffic electronics (distance warnings, measurement of ground speed) and in the consumer area (motion sensors, door openers), a minimization of technological outlay and thereby of cost is particularly important. A radar system particularly suited for these applications uses a monostatic construction in which one antenna is used both for transmission and for reception. A radar system of this sort is called monostatic, in contrast to systems with separate transmitting and receiving antennas, called bistatic radar. In the standard design of a continuous-wave radar system in conventional radar technology, a radio-frequency signal produced by an oscillator is both radiated via the antenna as a transmission signal and is also used to convert the reception signal to a considerably lower frequency (the difference frequency of the two signals) in a mixer, according to the homodyne principle. For this purpose, a part of the power given off by the oscillator is branched off from the transmission signal using a directional coupler, and is supplied to the input of the mixer.
A system of this sort is schematically illustrated in FIG. 3. The oscillator 1 supplies the radio-frequency signal, from which a part is branched off in the directional coupler 11 and is supplied to the mixer 13. The separation of the transmission and reception signal at the antenna 3 ensues by means of a circulator 12. According to the application of the system, the frequency of the radio-frequency signal remains constant (e.g. in Doppler radar), or the frequency is altered by an external modulation signal (FMCW radar frequency modulated continuous wave!, pulse FM radar). The terminal 4 provided for the feeding of the external modulation signal and the terminal 5 provided for the tapping of the intermediate frequency are also shown in FIG. 3.
The circulator 12 used in this arrangement is a substantial obstacle to a simple and economical manufacturing of the radar system. A construction that does not use this circulator can be most simply realized by using two separate antennas for transmission and reception (bistatic radar). The main disadvantage here is the increase in the outer dimensions, caused essentially by the antennas. In addition, the directional characteristics do not agree precisely, especially at close range, due to the spatial displacement of the antennas. The circulator can also be omitted in a monostatic radar system if for example the arrangement shown in FIG. 4 is chosen. In this arrangement, the directional coupler 14 is also used to conduct the reception signal coming from the antenna to the radio-frequency input of the mixer 13. Corresponding to the coupling attenuation of the directional coupler 14, the power of the reception signal is not conducted completely to the mixer, in contrast to the arrangement with the circulator. The lower receiver sensitivity that thereby results can mainly be tolerated in the applications mentioned above, due to the short range (200 m maximum).
In the arrangements described, it was presupposed that the mixer is provided with separate inputs for the local oscillator signal and for the radio-frequency reception signal. This is the case in constructions with balanced mixers, preferred in microwave technology. In a non-balanced mixer (e.g. a one-diode mixer), the two radio-frequency signals to be mixed are supplied via a common terminal. With the use of such a mixer, a further simplification of the system according to FIG. 4 is possible by replacing the directional coupler with a T branching. The essential disadvantage of this variant is the negative influence on the sensitivity, due to the amplitude noise of the oscillator, said noise being demodulated in the non-balanced mixer. This solution thus appears to be suited only for sensors with ranges of a few meters.